Extrusion vulcanized molded articles comprising rubber compound products of ethylene/propylene/nonconjugated diene ternary copolymers (EPDM) have been generally used in parts for which low hardness and rubber elasticity are required in the applications such as automobile parts, electric/electronic parts and construction parts.
In the meantime, thermoplastic elastomers requiring no vulcanizing process have started to be used in place of vulcanized rubbers using ethylene/propylene/nonconjugated diene ternary polymers (EPDM) for seal materials in various uses from the viewpoint of productivity, environmental correspondence and light-weight.
As technologies concerning the compositions of thermoplastic elastomers, there are known technologies used to dynamically crosslink an ethylene/propylene (/nonconjugated diene) copolymer with a crystalline polyolefin. As the crystalline polyolefin, an isotactic polypropylene is used in consideration of, particularly, product property and moldability.
However, conventional thermoplastic elastomers are generally inferior to vulcanized rubbers in rubber elasticity represented by compression set as an index and therefore cannot be said to be satisfactory. When the amount of a crosslinking agent and the like is increased to improve the rubber elasticity, there is the problem that the degree of crosslinking is raised, which impairs the fluidity of a molded article, leading to deteriorated appearance. There is also the problem that die-residue adheres to a dice during extrusion molding, impairing the outward appearance and only insufficient extrusion moldability is obtained.
On the other hand, thermoplastic elastomers using a syndiotactic polypropylene as a crystalline polyolefin are described in, for example, each publication of JP-A Nos. 6-287368, 7-247388 and 2000-355644.
It is an object of the present invention to solve the problems involved in the above prior art technologies and to provide a thermoplastic elastomer having higher extrusion moldability than conventional thermoplastic elastomers, and a molded article of the thermoplastic elastomer.